This study involves the direct observation of morphologic and functional aspects of isolated nephron segments in acute renal tubular necrosis and subsequent regeneration. The morphology will be characterized by direct observation by an inverted light microscope and ultrastructurally by electron microscopy. Physical parameters of the injured and regenerating nephron segments will be characterized as well as transport across the epithelium in these segments. These studies should provide basic information concerning the injury and recovery of renal tubular epithelium and should elucidate some of the problems involved in the kidneys of patients with acute renal failure.